1. Field of the Invention
The present invention relates to a diesel particulate removing apparatus and more particularly to a diesel particulate removing apparatus capturing particulates contained in an exhaust gas from an internal combustion engine such as a diesel engine by means of heaters and filters, burning the captured particulates, and removing the particulates from the filters to reuse or refresh the filters.
The present application claims priorities of Japanese Patent Applications Nos.2000-355793 filed on Nov. 22, 2000 and 2000-366412 filed on Nov. 30, 2000 which are hereby incorporated by reference.
2. Description of the Related Art
Nowadays, the object of restriction to an automobile exhaust gas is nitrogen oxides (NOx), containing mainly carbon and nitrogen, which is said to be a cancer generating substance.
Exhaust gas from diesel vehicles contains a number of carbon particulates (so called black fume or smoke). When the carbon particulates enter into the air through an exhaust duct, they float in the air for a long time, and finally drop onto house floors and roads or attach to clothing in a form of soot. According to recent medical reports, carbon substance absorbs various materials of considerable volume, chemical substances such as cancer-generating related ones are therefore adhered to the floating carbon particulates. The carbon particulates inhaled by human beings enter into their bodies and cause cancers and diseases of the respiratory system.
As described above, it is an important subject for study to restrict not only nitrogen oxides (NOx) but particulate matter (PM) exhausted from the diesel vehicles. In order to protect the environment from pollution from the black fume, a black fume capturing apparatus provide with a black fume removing filter consisting of metal fiber and honeycomb-shape elements, which is placed in the exhaust duct of the automobile diesel engine (see Japanese Utility model laid-open Sho61-55114 and Sho61-84851 official gazettes). However, when the black fume removing filter of this kind has been used for a long time, the filter clogs easily with the particulates of collected black fume, thus increasing disadvantageously a pressure loss.
As a resolving means for such clogging with black fume, diesel engine particulates removing apparatuses are proposed and described in the official gazettes of Japanese Patent Laid-open No. Hei2-173310, No. Hei6-212954, and No. Hei8-193509.
As shown in FIG. 16, the diesel engine particulates removing apparatus as mentioned above has a cylindrical vessel 3 with an inlet 1 and an outlet 2 for an exhaust gas, a ceramic heater 4 provided at an inlet side within the cylindrical vessel 3, a porous foam filter 5 placed adjacent to the back of the ceramic heater 4, and a catalyst mounting filter 6 arranged adjacent to the back of the porous foam filter 5.
According to the conventional structure of the diesel engine particulates removing apparatus as described above, an exhaust gas introduced into the cylindrical vessel 3 through the inlet 1 contacts with the ceramic heater 4 and burns. Unburned particles are captured in the porous foam filter 5 and the catalyst mounting filter 6 to remove the captured particulates. At this time, thermal energy or heat of the ceramic heater 4 and an exhaust gas conducts to the porous foam filter 5 to burn the unburned particles collected in the porous foam filter 5. The catalyst mounting filter 6 placed downstream of the ceramic heater 4 receives less conductive heat from the ceramic heater 4 and an exhaust gas, which lowers the temperature of the catalyst mounting filter 6. Catalyst mounted in the catalyst mounting filter 6 accelerates burning of collected particulates, so that many particles captured burn at a relatively low temperature and are removed without an accidental fire, resulting in recovery of filtering function of the catalyst mounting filter 6.
However, in accordance with the conventional apparatus for removing diesel engine particulates, since the catalyst mounting filter 6 is placed downstream of the ceramic heater 4, the temperature of the catalyst mounting filter 6 tends to become high at upstream side and low at downstream side. As a result, some particulates (fine particulates) may be left unburned, and the catalyst mounting filter 6 is clogged with the unburned particles, thus increasing a pressure loss. In some cases, a large amount of unburned and collected particulates in the catalyst mounting filter 6 are ignited and cause damage to the catalyst mounting filter 6.
In order not to leave unburned particulates in the catalyst mounting filter 6, there is a countermeasure to increase a heat or thermal energy of the ceramic heater 4 so as to raise a temperature at the rear end portion of the catalyst mounting filter 6 to an ignition temperature of the particulate. With this countermeasure, however, a temperature at upstream side of the ceramic heater 4 is raised to exceed its allowable temperature. This causes breaking of a wire and degradation in durability of the ceramic heater 4. According to the conventional apparatus as mentioned above, the ceramic heater 4 must be used by getting its heat generation under control. Consequently, there is a problem that catalyst placed in downstream end portion of the catalyst mounting filter 6 is not activated sufficiently, and unburned particulates remain in that portion.